A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of them have already been put into practical use. Widely employed among them is a system in which a photoreceptor comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles, e.g., zinc oxide, and a resin binder is subjected to an ordinary electrophotographic processing to form a highly lipophilic toner image thereon, followed by treating the surface of the photoreceptor with an oil-desensitizing solution referred to as an etching solution, to selectively render non-image areas hydrophilic and thus obtain an offset printing plate.
Requirements of offset printing plate precursors for obtaining satisfactory prints include: (1) an original should be reproduced faithfully on the photoreceptor; (2) the surface of a photoreceptor has affinity with an oil desensitizing solution so as to render non-image areas sufficiently hydrophilic, but, at the same time, has resistance to solubilization; and (3) a photoconductive layer having an image formed thereon is not released during printing and is well receptive to moistening water so that the non-image areas retain the hydrophilic properties sufficiently to be free from stains even upon printing a large number of prints.
It is known that these performance properties of the printing plate precursors are influenced by the ratio of zinc oxide to resin binder in the photoconductive layer. For example, as the ratio of resin binder to zinc oxide particles becomes small, oil-desensitivity of the surface of the photoconductive layer is increased to reduce background stains but, in turn, the internal cohesion of the photoconductive layer per se is weakened, resulting in reduction of printing durability due to insufficient mechanical strength. On the other hand, as the proportion of the resin binder increases, printing durability is improved, while background staining becomes conspicuous. With respect to background staining, while it is a phenomenon associated with the degree of oil-desensitization achieved, it has been elucidated that the oil-desensitization of the photoconductive layer surface depends not only on the zinc oxide/resin binder ratio in the photoconductive layer, but also depends greatly on the kind of the resin binder used.
Resin binders which have been conventionally known include silicone resins (see Japanese Patent Publication No. 6670/59), styrene-butadiene resins (see Japanese Patent Publication No. 1960/60), alkyd resins, maleic acid resins, polyamides (see Japanese Patent Publication No. 11219/60), vinyl acetate resins (see Japanese Patent Publication No. 2425/66), vinyl acetate copolymer resins (see Japanese Patent Publication No. 2426/66), acrylic resins (see Japanese Patent Publication No. 11216/60), acrylic ester copolymer resins (see Japanese Patent Publication Nos. 11219/60, 8510/61, and 13946/66), etc. However, electrophotographic light-sensitive materials using these known resins suffer from one or more of several disadvantages, such as low charging characteristics of the photoconductive layer, poor quality of a reproduced image (particularly dot reproducibility or resolving power), low sensitivity to exposure; insufficient oil-desensitization attained by oil-desensitization for use as an offset master (which results in background stains on prints when used for offset printing), insufficient film strength of the light-sensitive layer (which causes release of the light-sensitive layer during offset printing and failure to obtain a large number of prints), susceptibility of image quality to influences of environment at the time of electrophotographic image formation (such as high temperature and high humidity), and the like.
For particular use as an offset printing plate precursor, formation of background stains due to insufficient oil-desensitivity presents a serious problem. In order to solve this problem, various resins as binders for zinc oxide have been proposed, including a resin having a molecular weight of from 1.8.times.104 to 1.0.times.104 and a glass transition point of from 10 to 80.degree. C., obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in Japanese Patent Publication No. 31011/75; a terpolymer containing a (meth)acrylic ester unit having a substituent having a carboxylic group at least 7 atoms distant from the ester linkage as disclosed in Japanese Patent Application (OPI) No. 54027/78 (the term "OPI" as used herein means "unexamined published application"); a tetra- or pentamer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in Japanese Patent Application (OPI) Nos. 20735/79 and 202544/82; a terpolymer containing a (meth)acrylic ester unit having an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group as disclosed in Japanese Patent Application (OPI) No. 68046/83; and the like.
Nevertheless, evaluations of such resins as noted above for improving oil-desensitization indicate that none of them is completely satisfactory in terms of stain resistance, printing durability, and the like.